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These three thistle species are declared noxious in the Southern Tablelands and South East Region in all LCAs except for Bega Valley, Shoalhaven and Illawarra, in class 4.

Description

Most thistles are erect single-stemmed or branching biennial herbs ranging from 30cm to 2m high depending on species and growing conditions. They are characterised by having long spines on the leaf margins and stems. The plant begins life as a rosette, from which an elongated flowering stem generally arises. Flower heads consist of numerous small flowers clustered into cylindrical or hemispherical heads at the branch tips, and surrounded by spiny bracts. Flower colour is pink to purple for most of the species found in the region, except for a few with yellow flowers.

Scotch thistle (sometimes called cotton thistle) is distinctive in being covered in dense white woolly hairs, giving the leaves and stems a silver appearance. The stems are winged and flower heads pink, enclosed in many rows of spiny bracts. Black or spear thistle (Cirsium vulgare) is very frequently referred to as Scotch thistle but this is incorrect. Black thistle is deep green in colour, not silvery. Illyrian thistle is very similar to Scotch thistle, though less silvery. Its flower heads are enclosed in more flattened bracts. Stemless thistle is unusual in that it does not develop a tall flowering stem, but remains a rosette, with the flower heads in the centre. The flower is very similar to that of Scotch thistle. For additional thistle photos, check the entries for nodding thistle, star thistle and soldier thistle.

preferred habitat and impacts

Thistles are invasive weeds of pasture, reducing carrying capacity. The broad flat rosette habit in the early stages of growth smothers surrounding grass plants, and the density of stands which can occur after disturbance such as over-grazing or cultivation can choke out all other vegetation. Illyrian thistle produces particularly large rosettes. Unpalatable to stock because of the spines, thistles are favoured by heavy grazing. The spiny nature of thistle plants restricts stock and human movement in infested pasture.

Thistles are a more troublesome weed in the drier tablelands and slopes of southern NSW than on the coast, and more species are present in these areas. Black or spear thistle is the most common thistle found on both the coast and the tablelands, but other thistle species are quite uncommon on the coast. Scotch, Illyrian and stemless thistles are not listed as noxious in 3 coastal LCAs because they have not been recorded there as yet, but should be controlled if found. Thistles are also environmental weeds, invading grasslands and grassy woodlands.

dispersal

Seed may be wind-blown, and moved around in soil and on animals, vehicles and machinery. Contaminated hay and agricultural seed are also a source of infestations. The degree of wind movement of thistle seed varies with the species. Although all have a parachute of bristles to help keep the seed aloft, in some species this breaks off readily and does not assist much with dispersal. In the case of the Onopordum thistles, seed can drift for long distances on the wind.

look-alikes

All thistles are broadly similar in appearance and the distinguishing features of the three Onopordum species have been discussed above. Some plants have a resemblance to thistles although they are not closely related.The weed Mexican poppy (Argemone ochroleuca) has thistle-like spiny foliage which is silvery grey in colour, though not hairy like Scotch thistle. It is in the poppy family, and has a cream coloured poppy flower and a prickly seed capsule which splits at the top to release numerous small black seeds. It grows to about 1m high. Wild teazel (Dipsacus fullonum) has more elongated cylindrical heads of mauve flowers with long spiny bracts at the base, and short prickles on stems and the vein on the leaf underside, but not on leaf margins. It is also a weed. See the nodding thistle page for images of these thistle look-alikes.

There is a native plant with thistle-like foliage, the blue devil (Eryngium rostratum). It is a plant of native grasslands and grassy woodlands on the tablelands and slopes, and is very unlikely to be found on the coast. It has blue flowers in branched heads, and is not in the daisy family. Its leaves are green, not grey-green or silvery, and deeply lobed but not spiny. It is illustrated on the nodding thistle page.

control

Small infestations of thistles can be chipped out, but may regrow if the cut is not made deeply enough. Hold the top of the plant down to the ground with one foot to get the spiny leaves away from your hands while chipping, or catch them while still in the rosette stage, when they are very much easier to cut. Spot spraying or boom spraying can be used for larger infestations. Slashing can be a temporary measure to delay seeding, but plants will regrow from the base and viable seed may form on the cut plants if slashing is done after heads have been fertilised. Seed is long-lived in the soil (up to 10 years), so prevention of seeding is important.